I haven't looked into it, but from what I understand it won't/can't be as privileged as it is currently. It can likely be made to execute snippets of code but not on chrome or using xpcom apis, ie I doubt it can add an xul statusbar to the dom, etc. so won't be hugely useful. And the xul overlay part is surely dead. For web page manipulation, whatever GreaseMonkey can do is an alternative. I'm also unlikely to take the time to figure it out/convert it, sorry.

Thanks morat, .cfg is indeed a way to bypass being an xul overlay. However,1) a web extension would need to write to this file, this ability could be removed in the future.2) the code executed in .cfg could be limited to just the pref setting/locking for which the file was originally intended.